Surface waters, including lakes, rivers, wetlands, and all of their connecting waterways, can collect sulfates through geological weathering of local rock, atmospheric deposition from local and distant coal-burning power plants, and from activities that disturb and expose sulfur-bearing rock, like mining and roadbuilding. For example, many northeastern Minnesota water bodies contain sulfate concentrations that exceed current state regulatory levels, and the removal of excess sulfate and remediation of those sites is a paramount concern in Minnesota, among other places. For instance, native wild rice, Zizania palustris, grows well in water containing sulfate at concentrations at or below 10 mg/L but is relatively sparse in waters containing sulfate at 50 mg/L or more. While sulfate and other contaminants can be effectively removed from water by municipal or industrial waste water treatment plants, these systems are technologically complex, require high capital and operational costs, and demand continuous energy supplies. Notably, many of these waste treatment schemes include biological components, which use microbial metabolism to lower the concentrations of specific environmental contaminants, like phosphates, nitrates, or sulfates.
Given its reactivity, sulfur will rapidly participate in a variety of chemical reactions, depending on physico-chemical conditions, e.g. the presence of air (oxygen), water, and metals, and it is an active agent in the metabolism and structural biology of organisms ranging from bacteria to vertebrates. Therefore, it is not surprising that natural sulfur cycling through abiotic and biotic realms is usually environmentally benign and that a variety of stable ecosystems thrive in a wide range of sulfate levels, usually reflecting the underlying geology of the region. However, large releases of sulfur into the air through fossil fuel combustion, or into air or water from mining, road-building, and other processes that involve geological disturbance, can present a hazard. For example, when sulfur is exposed to air and water, sulfuric acid will form to some degree, which can reduce the pH of the terrestrial or aquatic matrix. Small amounts of acid produced by natural weathering are often neutralized by basic soil components or diluted by precipitation, and some fortunate regions, like northeast Minnesota, have sufficient geologic buffering capacity to eliminate most sulfate acidification. However, in other areas, exposure of large quantities and/or high concentrations of sulfur to air and water can generate sulfuric acid concentrations in precipitation and runoff that exceed natural buffering or dilution capacity. Acidified precipitation originating from coal-fired power plants and runoff from newly exposed rocks in mining operations has caused severe disruptions in sensitive aquatic and terrestrial ecosystems that have limited natural buffering capacity.
Sulfate is a significant issue for many mining operations. A specific type of sulfate problem is acute on the Iron Range in northeast Minnesota. Iron ore mining has been happening there for close to 200 years. Since the 1950's it has largely been a low grade of iron ore, taconite, that has been mined in this region. Taconite must be crushed and magnetically concentrated to form high iron content taconite pellets. The crushed taconite and the undesirable tailings are all transported as a water slurry. In the taconite ore there is a small quantity of metal sulfide ore, primarily pyrite. When in contact with water and air these sulfide ores oxidize to form sulfate in the water that is pumped out into the tailings basins. Sulfate concentration in the water is further elevated when a mining operation uses scrubbers to remove sulfur from their air emissions. The result is that all taconite processing plants in Minnesota have high sulfate concentrations to varying degrees in their tailings basins. Typical sulfate concentrations are from 500 to 2,000 mg/L.
The federal secondary drinking water standard for sulfates in the discharges or seepage from these tailings basins is 250 mg/L. However, if there are wild rice waters downstream of these seepages and discharges, then the applicable standard is 10 mg/L of sulfate in Minnesota. This is the special Minnesota standard for wild rice producing waters because of the damage that sulfate and/or hydrogen sulfide can cause to wild rice.
There is no taconite processing plant in Minnesota today that meets the wild rice standard of 10 mg/L of sulfate in their discharge water, and most do not meet the 250 mg/L standard either. This situation has been allowed to perpetuate until now based on the argument that there was no viable way to remedy the problem. Some have proposed just shutting down the taconite operations that provide 80% of the iron for steel making in the USA. This would be not only an economic disaster, but it would aggravate the sulfate problem further. When a taconite mine stops operating, and its sump pumps stop, the mine pits become mine pit lakes. In these mine pit lakes, water and air gradually oxidize the small concentration of pyrite in the exposed ore to form sulfate. Thus, the taconite mine pit lakes become water bodies high in sulfate. Even with this level of sulfate, these lakes do not typically form acid mine drainage in that they are well buffered with carbonates. This is evidenced by the many mine pit lakes with high sulfate that formed when Eire Mining ceased operations in 2001. These mine pit lakes have sulfate concentrations over 1,000 mg/L and a pH of 7 to 8. It is these waters that are discharged into natural streams that once had wild rice. This sulfate level is far above the Minnesota standard of 10 mg/L, but with no viable remediation available, no remedial action has been taken.
There are now advanced plans for the mining of sulfide ores (Cu, Ni, Ag, Au and others) very near to the taconite mines. PolyMet Mining hopes to be granted their Permit to Mine yet in 2018. For this they have committed to meet the 10 mg/L sulfate discharge limit. PolyMet plans to do this with reverse osmosis. They have proven that it is possible to meet the 10 mg/L standard, and they can afford to do it based on their high value ores.
The precedent has now been set by PolyMet that something can be done about high sulfate concentrations, albeit at an extremely high capital investment and operating cost. The legacy mine pit lakes and current taconite processing plants do not have sufficient revenue to justify reverse osmosis for the water currently being discharged. Several mines are threatening to cease operations if they are required to install and operate reverse osmosis systems for these applications. As mentioned hereinabove, if they do stop mining then the problem only gets worse.
There are many scholarly reports on the well-developed practice of bioremediation using bioreactors for the treatment of Acid Mine Drainage (AMD) or Acid Rock Drainage (ARD). Other commonly used bioreactors are in the form of Permeable Reactive Barrier (PRB) bioreactors to treat nitrates or sulfates. All these bioreactors fall into the two broad categories, namely: 1) organic media bioreactors where the support media and electron donor are the same material, and 2) those with inorganic support media and a separate liquid electron donor feed.
“Compost” bioreactors with organic media that also serve as electron donor are relatively inexpensive to build and relatively passive. But over time the support media is consumed and the electron donor supply decreases. As the organic media decomposes it loses its structure allowing for the forming of preferential flow paths and leads to plugging. At some point they generally need to be dug up and reconstructed.
Bioreactors with inorganic media, such as gravel, do not suffer from media degradation and therefore maintain more consistent flow patterns. For these bioreactors a liquid electron donor is generally used. The bacteria attach themselves to the available surface area of the media and consume the electron donor as it flows past them to convert sulfate to hydrogen sulfide. The hydrogen sulfide produced will react with dissolved metals to form metal sulfide precipitates. These precipitates can plug the bioreactor beds. However, with inorganic gravel media the precipitates can usually be flushed out of the system periodically to restore open flow patterns. Gravel media typically has about 150 m2/m3 of surface area and 50% void volume. The higher the surface area, the more sulfate-reducing bacteria (SRB) can grow in the bioreactor. The higher the void volume, the higher the retention time for sulfate reduction and the easier water flows through the media without biomass fouling.
The article “Passive Treatment of Acid Mine Drainage in Bioreactors using Sulfate-Reducing Bacteria, Critical Review and Research Needs” published in the Journal of Environmental Quality, Jan. 9, 2007 is an excellent summary of both types of bioreactor used in AMD applications. These applications normally have low pH due to the progressive oxidation of iron sulfide, or pyrite, producing progressively more acidity. This low pH further dissolves additional metals into the water stream. In these situations, bioreactors use SRB to increase the pH and to produce H2S to react with and precipitate out the dissolved metals (such as Cu2+, Zn2+, Cd2+, Pb2+, Ag 2+ and Fe2+). Another characteristic of these bioreactors is that they sequester sulfur in the form of metal sulfide thereby removing sulfur from the effluent water. This article focuses on “passive” treatment and covers primarily compost type bioreactors with organic media.
Another report entitled “Bioremediation of Acid Mine Drainage Using Sulfate-Reducing Bacteria” prepared by Sheela M. Doshi for the U.S. Environmental Protection Agency in 2006 describes the general state of the art at that time. Most of the bioremediation systems described in this report use organic substrate as a slow release electron donor as well as for SRB attachment. The organic substrate is consumed over time losing its structure and porosity. These applications have a low pH with high concentrations of dissolved metals. Therefore, as H2S is produced in the bioreactors it immediately reacts with the dissolved metal to form metal sulfide precipitates within the bed of the bioreactor. Over time the bioreactor loses its organic substrate attachment media and accumulates metal sulfide precipitates which require it to be cleaned out or reconstructed. One Modular Sulfate-Reducing Bioreactor Design is reviewed where bags of walnut shells are used as a very slow release electron donor and longer-term attachment surface for SRB in a format that can be more easily removed and replaced.
Another bioreactor approach reviewed in this report is the Leviathan Mine Compost-Free Bioreactor system. In this case ethanol was used as a liquid electron donor. Sodium hydroxide and a recycling mode were employed to raise the pH to near neutral pH to enhance SRB growth. With a liquid electron donor they could use an inorganic attachment media in the form of pea gravel. This gravel media had about 50% porosity and 150 m2/m3 of surface area. The use of this gravel media allowed the metal sulfides that were formed to flow out with the effluent water into a large settling pond for collection. The gravel media was periodically back flushed to eliminate any remaining metal sulfides that might cause fouling. Although the objective of this bioreactor system was not sulfate reduction, it did reduce dissolved metals, and raise pH while also reducing sulfate by about 17%. More importantly, it did demonstrate the workability of a gravel media, a liquid electron donor and the collection of the metal sulfide precipitates in a separate settling pond stage after flushing of the media bed.
In his report “Reduction of Sulfate Concentrations in Neutral Mine Effluent,” Glenn C. Miller, Ph.D. at the University of Nevada, Reno, Sep. 27, 2005, explains different approaches to sulfate reduction where the mine effluent is neutral due to sufficient carbonate to neutralize the sulfuric acid produced. Dr. Miller reviews chemical precipitation, ion exchange, reverse osmosis and membrane methods as well as biological sulfate reduction. He concludes that biological sulfate reduction can operate semi-passively and has relatively low capital and operational costs. His cost estimate to reduce sulfate concentrations from 1,500 mg/L to less than 250 mg/L is a cost of $5/1000 gallon not including hydrogen sulfide treatment.
One of the more recent reviews of biological sulfate reduction was a presentation titled “Sulfate-Reducing Bacteria: Current Practices and Perspectives” presented by P. Kousi, E. Remoundaki, A. Hatzikioseyain and M. Tsezos at the IWA Balkan Young Water Professionals, May 10-12, 2015. The conclusions of this review are that SRB can effectively reduce sulfate, reduce acidity and soluble metal species sequestration.
All these prior art bioreactor forms rely on either organic substrate for electron donors and attachment surface media, or a combination of low-cost gravel for attachment media and a liquid electron donor. The performance of these units is limited by the attachment media and substrate used. The organic media and substrate decomposes, is consumed, has changing permeability and must be exchanged after a few years. The gravel has a calculated porosity of about 50% and a limited surface area of about 150 m2/m3, which demands a very large volume bioreactor to achieve high flow rate and an acceptable retention time.
There is, therefore, a need for higher efficiency, lower cost, positively controllable sulfate reduction systems to address the sulfate issue for the taconite industry. Accordingly, it would be desirable to have a cost effective, self-contained means for use in the reduction of sulfates in surface water.